Microscopes employing conventional optical imaging systems cannot resolve features substantially smaller than about one-half an optical wavelength. That is, when both the entrance pupil of the microscope objective and its distance from the specimen are substantially larger than a wavelength, diffraction effects limit the smallest resolvable separation between a pair of object points to 0.5.lambda./N.A., where .lambda. is the optical wavelength and N.A. denotes the numerical aperture of the objective, which, as a practical matter, is limited to values of about 1.6 or less. (F. A. Jenkins and H. E. White, Fundamentals of Optics, Third Edition, McGraw-Hill Book Co., New York, 1957, pp. 306-308.)
Electron microscopy has been successful in resolving features much smaller than those resolvable with conventional optical microscopy. However, for some applications, electron microscopy suffers from certain disadvantages. For example, the specimen must be enclosed in an evacuated chamber. Where the electron microscope is employed as a diagnostic device on a production line for, e.g., semiconductor wafers, the time required to vent and evacuate the chamber may detract significantly from the manufacturing throughput. As another example, certain features of a specimen that are detectable by optical microscopy may nevertheless be invisible to an electron microscope, because different contrast mechanisms are involved. As yet another example, the vacuum environment, or exposure to the electron beam, may be destructive to the specimen.
A number of researchers have investigated the use of optical scanning to circumvent the inherent limitations of conventional optical imaging systems. That is, in so-called near-field scanning optical microscopy (NSOM), an aperture having a diameter that is smaller than an optical wavelength is positioned in close proximity to the surface of a specimen, and scanned over the surface. In one scheme, the specimen is reflectively or transmissively illuminated by an external source. A portion of the reflected or transmitted light is collected by the aperture and relayed to a photodetector by, for example, an optical fiber. In an alternate scheme, light is relayed by an optical fiber to the aperture, which itself functions as miniscule light source for reflective or transmissive illumination of the specimen. In that case, conventional means are used to collect and detect the selected or transmitted light. In either case, the detected optical signal is reconstructed to provide image information.
Thus, for example, U.S. Pat. No. 4,604,520, issued to W. D. Pohl on Aug. 5, 1986, describes an NSOM system using a probe made from a pyramidal, optically transparent crystal. An opaque metal coating is applied to the crystal. At the apex of the crystal, both the tip of the crystal and the metal coating overlying the tip are removed to create the aperture, which is essentially square and has a side length less than 100 nm.
Also described in U.S. Pat. No. 4,604,520 is an alternative aperture made from a single-mode optical fiber. One planar end of the fiber is metallized, and a coaxial hole is formed in the coating so as to just expose the core of the fiber.
In a somewhat different approach, U.S. Pat. No. 4,917,462, issued to A. L. Lewis, et al. on Apr. 17, 1990, describes a probe formed from a pipette, i.e., a glass tube that is drawn down to a fine tip and coated with an opaque metal layer. After drawing, the pipette retains a hollow bore, which emerges through both the glass and the overlying metal layer at the tip. The resulting metal annulus defines the aperture. The aperture may be smaller than the bore defined in the glass, as a result of radially inward growth of the metal layer.
In yet another approach, R. C. Reddick, et al., "New form of scanning optical microscopy," Phys. Rev. B, 39 (1989) pp. 767-770, discusses the use of a single-mode optical fiber as a probe for so-called photon scanning tunneling microscopy (PSTM). The fiber tip is sharpened by etching, and the tip is optionally coated with an opaque material to define an aperture on the very end of the fiber tip. (It should be noted that PSTM differs from transmission or reflection microscopy in that the illumination system is adapted to produce total internal reflection of the PSTM specimen. The probe tip is brought into the evanescent optical field above the sample. A portion of the optical energy in the evanescent near field is coupled into the probe and propagates through it, ultimately reaching a detector.)
One drawback of most of the above-described methods is that light is transmitted through the probe with relatively low efficiency. As a consequence, signal levels are relatively low. In some cases, apertures must be made larger in order to compensate for low signal levels. This measure is undesirable because it results in lower resolution. (PSTM generally offers relatively high signal levels, but resolution is generally no better than can be achieved by conventional optical microscopy.)
For example, when light is transmitted from a source to the aperture through an uncoated pipette, the optical field has a substantial amplitude at the outer walls of the pipette. In order to confine radiation, it is necessary to coat the walls with metal. However, attenuation occurs as a result of absorption in the metal coating. Moreover, metal coatings are prone to imperfections, such as pinholes, that permit optical leakage. When this tendency is countered by increasing the metal thickness, the length (i.e., the thickness in the axial direction of the pipette) and outer diameter of the metal annulus defining the aperture are also increased. As a result, optical losses due to absorption and evanescence in the metal annulus are increased and the size of the tip increased. Enlarging the tip makes it more difficult to scan narrow concave topographical features of the specimen while maintaining close proximity to the specimen surface. (Significantly, the problem of excessive tip size due to metal deposition also applies to constant-diameter optical fiber probes of the type having an aperture defined by a hole in a metal layer coating the end of the fiber.)
Analogous problems occur when light is transmitted in the opposite direction, i.e., from the specimen (by transmission or reflection) into the aperture, and thence through the pipette toward a detector. The optical signal is attenuated in the aperture region, as described. A portion of the optical signal may be lost by absorption in the metal coating of the walls, and through pinhole leaks in the metal coating, also as described. Moreover, scattered light may enter the pipette through pinhole leaks, resulting in an increased noise level at the detector.
Still further problems occur because a portion of the light that passes through the pipette toward the aperture is reflected from the outer glass walls of the pipette. After suffering multiple reflections, some of the light may undergo a reversal of propagation direction. As a consequence, the amount of light incident on the specimen may be reduced.
Probes made from pyramidal crystals suffer difficulties that are analogous to those described above in connection with pipette-type probes.
Problems that occur when the probe is a single-mode fiber having a sharpened (e.g., by etching) conical tip and no metal coating are described, e.g., in C. Girard and M. Spajer, "Model for reflection near field optical microscopy," Applied Optics, 29 (1990) pp. 3726-3733. One problem is that a portion of the light passing through the fiber toward the tip may be reflected by, and then transmitted through, the sides of the conical taper. A second problem is that the sides of the taper may capture undesired optical signals that can propagate through the fiber, resulting in an increased noise level at the detector.
In view of the foregoing discussion, it is apparent that investigators have hitherto been unsuccessful in providing an NSOM probe that combines efficient transmission of light (i.e., transmission that is relatively free of attenuation due to optical interactions with the walls of the probe) with relatively small tip dimensions, high resolution, and high reliability.